1. Field of the Invention
This invention pertains to optical sensors, and, more particularly, to protection of optical detectors in an optical sensor from damage by radiation in the pass band and field of view of the sensor's detector.
2. Description of the Related Art
Optical sensors are designed to receive and monitor relatively weak optical signals, whether those optical signals are natural or man-made. Thus the sensor's detectors are very sensitive and are therefore vulnerable to damage by high-level radiation, particularly if the radiation source is in the field of view and in the pass band of the sensor's focusing optics. For some applications, the optical detectors in a sensor must be protected from optical signals that are sufficiently strong to damage the detector. The most extreme example is found in military applications. Many military systems employ optical sensors for a variety of tasks. Enemy forces frequently employ counter-measures to incapacitate or damage the sensor with strong optical signals specifically designed to damage sensor(s). For instance, an enemy might illuminate an infrared imager with a high intensity laser capable of damaging the optical detector(s) in the imager. Sensors have been protected from in-band, in-view threats to some extent by mechanical shutters, reflective (notch filter) coatings, notch absorption materials, non-linear distortion and dispersion in a fluid cell, thermochromic elements, two-photon absorption materials and other techniques.
In the IR wavelengths, a thermoreflectance or thermochromic non-linear material (“NLM”) like Vanadium Dioxide (“VO2”) can be used to modulate radiation almost 100%. This concept has been extended to optical protection and limiting by subsequent research. For example, one protection approach coats the front surface of transmissive optical elements with VO2. In this approach, one of two NLM coated element is placed near a focal surface—typically a plane—through which the optical energy passes on its way to a sensor's detector(s). Below the “switching threshold,” the thermochromic NLM is transmissive to optical energy in the pass band of the sensor, that is, it transmits the “normal” optical energy incident upon it. However, above this threshold of irradiance, the NLM becomes reflective; i.e., it is opaque to the potentially damaging optical irradiance.
In the case of VO2, this optical effect is due to a change in the crystal structure and optical characteristics of the material that occurs when the thin film is above a critical temperature. Since temperature is a function of, among other things, the intensity with which the incident energy impinges on the NLM, the coating acts to limit incident radiation transmitted to the sensor detector(s). This intensity is called the “switching intensity”; i.e., the intensity which produces the temperature at which the thermochromic NLM switches from high to low transmission of the incident energy.
In operation, the thermochromic NLM remains transmissive for the optical energy impinging upon it that is within the desired bandwidth and intensity for the optical elements associated therewith. The optical elements behind the NLM and the substrate are thereby able to receive the incident optical energy. When optical energy of dangerous intensity (e.g., a high-powered laser threat) is encountered, the NLM heats up and switches to its reflective state, whereupon the high intensity optical energy is primarily reflected. When the dangerous intensity ceases, the NLM cools down and returns to its transparent, transmissive state. Thus, by reflecting dangerous intensities of optical energy, the NLM protects downstream optical elements (e.g., sensitive detectors) from damage.
Such thermochromic NLM coatings are however also subject to damage from sufficiently intense radiation. If the incident energy is sufficiently intense and of sufficient duration, the energy can melt, vaporize, or delaminate the NLM from its substrate. This degree of intensity is called the “damage threshold.” Thus, a NLM protected system whose optical detector(s) remain unharmed by the damaging intensity can still be degraded. To address this issue, a second NLM switch may then placed forward of the first to protect the first element from damage (although this results in some degradation of the sensitivity of the sensor).
One performance characteristic used to assess an optical protection apparatus is its “dynamic range.” The dynamic range is the ratio of its switching threshold to its damage threshold. Ideally, the damage intensity should be very large relative to the switching intensity, and so a large dynamic range is desirable. The desire to improve dynamic range for these materials continues to spur efforts at improving the design of reflective limiters employing thermochromic NLMs.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.